The tensile strength of concrete is lower than its compressive strength by a factor of approx. 10. The failure is effected in a relatively brittle manner. Concrete therefore needs to be reinforced when accommodating tensile forces or shear forces on the building construction. Safety considerations are often at the forefront. A concrete component on exceeding the maximal load should not break in a catastrophic and abrupt manner, but firstly absorb a certain amount of energy, thus display a ductile behavior. Both are conventionally achieved by way of steel reinforcement. The type of reinforcement is planned in a detailed manner and then the reinforcements are applied in costly manner mostly by hand. In certain cases, one may do away with conventional steel reinforcement completely or partly by way of admixing shortly cut steel fibers. Steel fibers however have decisive disadvantages. They are prone to corrosion which often leads to ugly strips of rust or spots on the concrete. They further entail the danger of injury. They also have poor dosing and mixing properties as a result of their rigidity. Large dosing weights arise as a result of the large density, witch shows up in the costs. The admixing of steel fibers also leads to a relative high scatter of the material properties due to the non-uniform distribution. Other fiber types, such as glass fibers, have decisive disadvantages, for example a limited resistance to alkali.
The use of plastic fibers offers an alternative. The fibers thereby on the one hand need to have a relatively large tensile strength, and on the other hand need to have a high bonding strength with the concrete. In the case of loading, the static friction on the complete surface of the fiber is to remain effective, so that the fiber is uniformly pulled out and is in the position of absorbing a lot of failure energy. Inexpensive fiber types, in particularly also compared directly to steel fibers, may be manufactured on the basis of polyolefin's (polypropylene, polyethylene) or other thermoplastic plastics. Whilst one succeeds with this, in achieving notable tensile strengths with values which to some extent are better than steel, the modulus of elasticity and the bonding strength to concrete is generally low with these types of fibers. An improvement of the static friction may be achieved by increasing the E-modulus of the fibers, manufactured of relatively expensive raw materials.
Multi-layered, thermoplastic plastic fibers for reinforcing concrete are known from EP 1350773. There, it is particularly emphasized that the polymers of the different layers have different melting points. The polymer with the lower melting point lies in the core, that with the higher one lies in the casing, wherein the difference is to be 10° C. to 20° C. This measure is to serve for the stretching after heating in a special oven, in that the inner layer is likewise adequately heated with the heating of the outer layer, so that a stretching by the factor 3 to 12 is possible. By way of the stretching, the plastic molecules are orientated longitudinally. The strength in the plastic is firstly achieved by way of this. These plastic fibers are provided on their outer side with structures before the stretching, for increasing the adhesive force in the concrete. In detail, these filaments are manufactured such that a double-layered or multi-layered film is created by way of co-extrusion. Afterwards, this film is provided with an embossing by way of calendar. The film is subsequently cut into narrow strip lets. At the end, the two-dimensional striplets are yet stretched, by which means the bulges or thin- and thick locations effectively arise.
It is however important to ascertain that the embossing and stretching take place in a coherent process. Partial material accumulations arise due to the embossing in the undrawn condition, as is taught by EP 1350773 A2. The polymer which is displaced by the structuring is still amorphous. If one draws thereafter, then firstly the zone with the smallest material accumulation is stretched. It is generally known and also obvious that firstly the locations with smallest resistance are drawn during each stretching process. In this case, this is clearly the thin locations. For this reason previously embossed striplets may not be uniformly drawn at the end, thus after a stretching after the effected embossing. It would be difficult or even impossible whilst maintaining favorable production conditions, which means whilst avoiding filament breakages, to be able to completely stretch filaments embossed in such a manner at all. The thin locations would be completely stretched, whilst at the thick locations, the degree of stretching and thus also the orientation of the molecules must necessarily be smaller. By way of this, the bulges which are manufactured according to this method are softer than the other locations of the filament, and accordingly they have an insufficiently high modulus of elasticity. This means that the bulges—on pulling out—are slightly worn. Furthermore, it is basically not possible to obtain sharp-edged bulges by way of stretching after the embossing, since the profile of the bulges are “blurred” due to the drawing, which is clearly evident in FIG. 1 of EP 1,350,773 A2. Since each chain is only as strong as its weakest member, this method also entails a certain amount of material wastage, since an over-proportional amount of polymer needs to be used, in order to achieve the designed strength values in the thin locations. FIG. 1(A) in EP 1,350,773 A2 likewise makes this point clear. With the embossing before the stretching, one may only achieve bulges which have a very large distance to one another. Thus in [0041] of EP 1350773 A2 it is mentioned that the stretch ratio is to be between 3:1 to 12:1 and preferably between 5:1 to 10:1. With a pyramid embossing as is shown in FIG. 1 (B), and after a subsequent minimal total drawing (stretch factor) of 5, the distances from bulge to bulge is 5 mm, with a total drawing of 10, which represents the absolute minimum with a high-strength PP or HDPE-filament, thus a distance of 10 mm from bulge to bulge results, without taking into account the embossing gap! FIG. 1 (A) of EP 1350773 A2 furthermore very clearly illustrates the profile of the filaments which one obtains by way of an embossing before the stretching. The fibers are above-averagely thin in the web (thin locations). The thickenings continuously increase towards the bulges (thick locations) and after become increasing flatter. Thus to a certain extent a cone is formed on both sides by each bulge. This particularity is always repeated with the method as is described under EP 1,350,773 A2, independently of which embossing type is selected, whether a pyramid, wave or angular profile, or a single-side or double-sided embossing. The bulges necessarily and always on both sides run out at a very acute angle to the diameter of the next thin location. The sliding out of the concrete, in comparison to a sharp embossing with marked transitions from the thin to the thick locations, is significantly more unfavorable.
Thus one may only achieve laterally flatted or rounded bulges which have a large distance to one another, with the embossing before the stretching, and furthermore it is clear that a structure conversion in the inside of the fibers is accepted as a result of the stretching which follows the embossing with regard to time. During each stretching process, firstly the locations with the smallest resistance are drawn. This in this case is clearly the thin locations produced on account of the structuring. For this reason, the fibers stretched after an embossing no longer have homogeneous molecule structures. Rather, the thin locations are completely stretched, whilst the degree of stretching at the thick locations and thus also the uniform orientation of the molecules is inevitably smaller. For this reason, an over-proportional amount of polymer is used, in order to achieve the desired strength values in the thin locations. Furthermore, the thick locations are soft, which likewise worsens the bonding to the concrete, and leads to a sliding out of the cement stone matrix, which is much more likely when compared to a hard polymer surface.